Golf ball

ABSTRACT

An object of the present invention is to provide a golf ball traveling a great flight distance and having a good shot feeling on long iron shots. The present invention provides a golf ball comprising a core, at least one intermediate layer covering the core and a cover covering the intermediate layer, wherein the core is formed from a core rubber composition containing (a) a base rubber containing a polybutadiene, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) a crosslinking initiator, and (d) a terpene-based resin, and a material hardness (Hm) of the intermediate layer is higher than a material hardness (Hc) of the cover, and the material hardness (Hc) of the cover is less than 50 in Shore D hardness.

FIELD OF THE INVENTION

The present invention relates to a golf ball, and particularly relatesto a technology for improving a core.

DESCRIPTION OF THE RELATED ART

It goes without saying that not only the flight distance on driver shotsbut also the precision on approach shots are important in making a goodscore when playing golf. A golf ball excellent in the performance onabout 20 yard-approach shot from around the green is required. Further,highly skilled golfers are interested in shot feeling on approach shotsfrom around the green.

A technique for improving the performance on approach shot from aroundthe green has been proposed by improving the paint film of the golfball. For example, Japanese Patent Publication No. 2014-14383 Adiscloses a golf ball comprising a golf ball body and a paint filmformed on a surface of the golf ball body, wherein the paint film has astorage modulus (E′) of 1.00×10⁷ dyn/cm² or more and 1.00×10⁸ dyn/cm² orless in a temperature range from 120° C. to 150° C., and a loss tangent(tan δ) of 0.050 or more at 10° C., measured with a dynamicviscoelasticity measuring apparatus under the following measuringcondition:

-   -   Measuring mode: tensile mode    -   Measuring temperature: from −50° C. to 150° C.    -   Temperature rising rate: 4° C./min    -   Oscillation frequency: 10 Hz    -   Measuring strain: 0.1%.

JP2016-93386A discloses a golf ball comprising a golf ball body and apaint film formed on a surface of the golf ball body, wherein the paintfilm is formed from a curing type paint composition including: a basematerial containing a polyrotaxane, and a curing agent containing apolyisocyanate, and wherein the polyrotaxane has a cyclodextrin, alinear molecule included in a cyclic structure of the cyclodextrin in amanner of piercing through the cyclic structure of the cyclodextrin, andhaving blocking groups located at both terminals of the linear moleculeto prevent disassociation of the cyclodextrin, and at least a part ofhydroxyl groups of the cyclodextrin is modified with a caprolactonechain via —O—C₃H₆—O— group.

SUMMARY OF THE INVENTION

Although improving the performance on approach shots has been addressedby improving the paint film of the golf ball, there is a limit onimproving the paint film. The inventors of the present invention havefound another approach to improve the performance on approach shots fromaround the green, which is a different way from improving the paintfilm.

An object of the present invention is to provide a novel golf ballhaving a high spin rate and a good shot feeling on approach shots.Especially, in the present invention, the object of the presentinvention is to provide a golf ball having a high spin rate and a goodshot feeling on about 20-yard approach shots.

The present invention provides a golf ball comprising a core, at leastone intermediate layer covering the core and a cover covering theintermediate layer, wherein the core is formed from a core rubbercomposition containing (a) a base rubber containing a polybutadiene, (b)an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof as a co-crosslinking agent, (c) a crosslinkinginitiator, and (d) a terpene-based resin, and a material hardness (Hm)of the intermediate layer is higher than a material hardness (Hc) of thecover, and the material hardness (Hc) of the cover is less than 50 inShore D hardness.

The gist of the present invention resides in blending a terpene-basedresin in the core of the golf ball having the specific construction toprovide a golf ball having a high spin rate and a good shot feeling onapproach shots from around the green.

The present invention provides a golf ball a high spin rate and a goodshot feeling on approach shots.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a partially cutaway cross-sectional view of a golf ballaccording to one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a golf ball comprising a core, at leastone intermediate layer covering the core and a cover covering theintermediate layer, wherein the core is formed from a core rubbercomposition containing (a) a base rubber containing a polybutadiene, (b)an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof as a co-crosslinking agent, (c) a crosslinkinginitiator, and (d) a terpene-based resin, and a material hardness (Hm)of the intermediate layer is higher than a material hardness (Hc) of thecover, and the material hardness (Hc) of the cover is less than 50 inShore D hardness.

First, the materials used for the core rubber composition are explained.

[(a) Base Rubber]

(a) The base rubber used for the core rubber composition according tothe present invention preferably contains a polybutadiene. As thepolybutadiene, a high-cis polybutadiene having a cis-1,4 bond in anamount of 90 mass % or more (hereinafter sometimes simply referred to“high-cis polybutadiene”) is preferable. The high-cis polybutadienepreferably has a 1,2-vinyl bond in an amount of 2.0 mass % or less, morepreferably 1.7 mass % or less, and even more preferably 1.5 mass % orless. This is because if the amount of the 1,2-vinyl bond is excessivelyhigh, the resilience may be lowered.

The high-cis polybutadiene is preferably one synthesized using arare-earth element catalyst. When a neodymium catalyst employing aneodymium compound which is a lanthanum series rare-earth elementcompound, is used, a polybutadiene rubber having a high amount of thecis-1,4 bond and a low amount of the 1,2-vinyl bond is obtained with anexcellent polymerization activity, and thus such polybutadiene rubber isparticularly preferred.

The polybutadiene preferably has a Mooney viscosity (ML₁₊₄ (100° C.)) of30 or more, more preferably 32 or more, and even more preferably 35 ormore, and preferably has a Mooney viscosity (ML₁₊₄ (100° C.)) of 140 orless, more preferably 120 or less, even more preferably 100 or less, andmost preferably 80 or less. It is noted that the Mooney viscosity (ML₁₊₄(100° C.)) in the present invention is a value measured according to JISK6300 using an L rotor under the conditions of preheating time: 1minute, rotor rotation time: 4 minutes, and temperature: 100° C.

The polybutadiene preferably has a molecular weight distribution Mw/Mn(Mw: weight average molecular weight, Mn: number average molecularweight) of 2.0 or more, more preferably 2.2 or more, even morepreferably 2.4 or more, and most preferably 2.6 or more, and preferablyhas a molecular weight distribution Mw/Mn of 6.0 or less, morepreferably 5.0 or less, even more preferably 4.0 or less, and mostpreferably 3.4 or less. This is because if the molecular weightdistribution (Mw/Mn) of the polybutadiene is excessively low, theprocessability may deteriorate, and if the molecular weight distribution(Mw/Mn) of the polybutadiene is excessively high, the resilience may belowered. It is noted that the molecular weight distribution is measuredby gel permeation chromatography (“HLC-8120GPC” available from TosohCorporation) using a differential refractometer as a detector under theconditions of column: GMHHXL (available from Tosoh Corporation), columntemperature: 40° C., and mobile phase: tetrahydrofuran, and calculatedby converting based on polystyrene standard.

From the viewpoint of obtaining a core having higher resilience, theamount of the polybutadiene in the base rubber is preferably 60 mass %or more, more preferably 80 mass % or more, and even more preferably 90mass % or more. It is also preferred that (a) the base rubber consistsof the polybutadiene.

(a) The base rubber may further contain another rubber in addition tothe polybutadiene rubber. Examples of another rubber include naturalrubber, polyisoprene rubber, styrene polybutadiene rubber, andethylene-propylene-diene rubber (EPDM). These rubbers may be used solelyor in combination of at least two of them.

[(b) Co-Crosslinking Agent]

(b) The α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or the metal salt thereof used in the core rubber composition isblended as a co-crosslinking agent in the rubber composition, and has anaction of crosslinking a rubber molecule by graft polymerization to abase rubber molecular chain.

Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms include acrylic acid, methacrylic acid, fumaric acid, maleic acid,and crotonic acid.

Examples of the metal constituting the metal salt of the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms include a monovalent metalion such as sodium, potassium, and lithium; a divalent metal ion such asmagnesium, calcium, zinc, barium, and cadmium; a trivalent metal ionsuch as aluminum; and other metal ions such as tin, and zirconium. Themetal component may be used solely or as a mixture of at least two ofthem. Among them, as the metal component, the divalent metal such asmagnesium, calcium, zinc, barium, and cadmium is preferred. This isbecause use of the divalent metal salt of the α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms easily generates a metal crosslinkingbetween the rubber molecules. Especially, as the divalent metal salt,zinc acrylate is preferred, because zinc acrylate enhances theresilience of the resultant golf ball. The α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and/or the metal salt thereof may beused solely or in combination of at least two of them.

The amount of (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or the metal salt thereof is preferably 15 parts bymass or more, more preferably 20 parts by mass or more, and even morepreferably 25 parts by mass or more, and is preferably 50 parts by massor less, more preferably 45 parts by mass or less, and even morepreferably 35 parts by mass or less, with respect to 100 parts by massof (a) the base rubber. If the amount of (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereofis less than 15 parts by mass, the amount of (c) the crosslinkinginitiator which will be explained below must be increased in order toobtain an appropriate hardness of the core formed from the core rubbercomposition, which tends to lower the resilience of the obtained golfball. On the other hand, if the amount of the α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and/or the metal salt thereof is morethan 50 parts by mass, the core formed from the core rubber compositionbecomes so hard that the shot feeling of the obtained golf ball may belowered.

[(c) Crosslinking Initiator]

(c) The crosslinking initiator used in the core rubber composition isblended in order to crosslink (a) the base rubber component. As (c) thecrosslinking initiator, an organic peroxide is preferred. Specificexamples of the organic peroxide include an organic peroxide such asdicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butylperoxide. Theseorganic peroxides may be used solely or in combination of at least twoof them. Among them, dicumyl peroxide is preferably used.

The amount of (c) the crosslinking initiator is preferably 0.2 part bymass or more, more preferably 0.5 part by mass or more, and even morepreferably 0.7 part by mass or more, and is preferably 5.0 parts by massor less, more preferably 2.5 parts by mass or less, and even morepreferably 2.0 parts by mass or less, with respect to 100 parts by massof (a) the base rubber. If the amount of the crosslinking initiator isless than 0.2 part by mass, the core formed from the core rubbercomposition becomes so soft that the resilience of the obtained golfball may be lowered. If the amount of (c) the crosslinking initiator ismore than 5.0 parts by mass, the amount of (b) the co-crosslinking agentwhich has been explained above must be decreased in order to obtain anappropriate hardness of the core formed from the core rubbercomposition, which may lower the resilience of the obtained golf ball orworsen the durability of the obtained golf ball.

[(d) Terpene-Based Resin]

The terpene-based resin used in the present invention is notparticularly limited, as long as it is a polymer having a terpenecompound as a constituent component. The terpene-based resin ispreferably, for example, at least one member selected from the groupconsisting of a terpene polymer, a terpene-phenol copolymer, aterpene-styrene copolymer, a terpene-phenol-styrene copolymer, ahydrogenated terpene-phenol copolymer, a hydrogenated terpene-styrenecopolymer, and a hydrogenated terpene-phenol-styrene copolymer.

The terpene polymer is a homopolymer obtained by polymerizing a terpenecompound. The terpene compound includes a hydrocarbon represented by acomposition of (C₅H₈)_(n) and an oxygen-containing derivative thereof,and is a compound having a terpene such as monoterpene (C₁₀H₁₆),sesquiterpene (C₁₅H₂₄) or diterpene (C₂₀H₃₂) as a basic skeleton.Examples of the terpene compound include α-pinene, β-pinene, dipentene,limonene, myrcene, alloocimene, ocimene, α-phellandrene, α-terpinene,γ-terpinene, terpinolene, 1,8-cineole, 1,4-cineole, α-terpineol,β-terpineol, and γ-terpineol. The terpene compound may be used solely orused as a mixture of two or more of them.

The terpene polymer is obtained, for example, by polymerizing the aboveterpene compound. Examples of the terpene polymer include α-pinenepolymer, β-pinene polymer, limonene polymer, dipentene polymer, andβ-pinene/limonene polymer.

The terpene-phenol copolymer (hereinafter sometimes simply referred toas “terpene phenolic resin”) is, for example, a copolymer of the aboveterpene compound and a phenol-based compound. Examples of thephenol-based compound include phenol, cresol, xylenol, catechol,resorcin, hydroquinone, and bisphenol A. As the terpene-phenolcopolymer, the copolymer of the above terpene compound and phenol ispreferable.

The acid value of the terpene-phenol copolymer is preferably 10 mgKOH/gor more, more preferably 35 mgKOH/g or more, and even more preferably 60mgKOH/g or more. In addition, the acid value of the terpene-phenolcopolymer is preferably 300 mgKOH/g or less, more preferably 250 mgKOH/gor less, even more preferably 200 mgKOH/g or less, particularlypreferably 150 mgKOH/g or less, and most preferably 90 mgKOH/g or less.It is noted that, in the present invention, the acid value of theterpene-phenol copolymer is an amount in milligram of potassiumhydroxide required for neutralizing the acid included in one gram of theterpene-phenol copolymer, and is a value measured by a potentiometrictitration method (JIS K 0070: 1992).

The hydroxy value of the terpene-phenol copolymer is preferably 30mgKOH/g or more, more preferably 50 mgKOH/g or more. The hydroxy valueof the terpene-phenol copolymer is preferably 150 mgKOH/g or less, morepreferably 100 mgKOH/g or less. It is noted that, in the presentspecification, the hydroxy value is an amount in milligram of potassiumhydroxide required for neutralizing acetic acid bonding to hydroxylgroup when one gram of the resin is acetylated, and is a value measuredby a potentiometric titration method (JIS K 0070: 1992).

The terpene-styrene copolymer is, for example, a copolymer of the aboveterpene compound and a styrene-based compound. Examples of thestyrene-based compound include styrene, and α-methylstyrene. As theterpene-styrene copolymer, the copolymer of the above terpene compoundand a-methylstyrene is preferable.

The terpene-phenol-styrene copolymer is, for example, a copolymer of theabove terpene compound, the above phenol-based compound and the abovestyrene-based compound. As the terpene-phenol-styrene copolymer, thecopolymer of the above terpene compound, phenol and α-methylstyrene ispreferable.

The hydrogenated terpene-phenol copolymer is one obtained byhydrogenating the above terpene-phenol copolymer. The hydrogenatedterpene-styrene copolymer is one obtained by hydrogenating the aboveterpene-styrene copolymer. The hydrogenated terpene-phenol-styrenecopolymer is one obtained by hydrogenating the aboveterpene-phenol-styrene copolymer.

As (d) the terpene-based resin, at least one member selected from thegroup consisting of compounds having a structure represented by thefollowing formulae (1) to (4) is preferable.

[In the formulae (1) to (4), R¹ and R² each independently represent adivalent residue of a phenol-based compound and/or a styrene-basedcompound, m¹ to m⁴ each independently represent a natural number of 1 to30, and n¹ to n² each independently represent a natural number of 1 to20.]

The compounds having the structure represented by the above formulae (1)to (4) each have a structure derived from pinene in the molecule.

The compound having the structure represented by the formula (1) has arepeating unit consisting of a structural moiety derived from α-pineneand R¹ bonding to the structural moiety derived from α-pinene. R¹ ispreferably a divalent residue where two hydrogen atoms are removed frombenzene ring of a phenol-based compound and/or a styrene-based compound.Examples of the compound having the structure represented by the formula(1) include a copolymer of α-pinene and a phenol-based compound and/or astyrene-based compound.

Examples of the phenol-based compound include phenol, cresol, xylenol,catechol, resorcin, hydroquinone, and bisphenol A. Examples of thestyrene-based compound include styrene, and a-methylstyrene.

In the formula (1), m¹ represents the degree of polymerization of thestructural unit derived from α-pinene, and is preferably a naturalnumber of 1 to 30. m¹ is preferably 1 or more, more preferably 2 ormore, and even more preferably 3 or more, and is preferably 30 or less,more preferably 25 or less, and even more preferably 20 or less.

In the formula (1), n¹ represents the degree of polymerization of therepeating unit consisting of the structural moiety derived from α-pineneand R¹ bonding to the structural moiety derived from α-pinene, and ispreferably a natural number of 1 to 20. n¹ is preferably 1 or more, morepreferably 2 or more, and even more preferably 3 or more, and ispreferably 20 or less, more preferably 18 or less, and even morepreferably 15 or less.

The compound having the structure represented by the formula (2) has arepeating unit consisting of a structural moiety derived from β-pineneand R² bonding to the structural moiety in the molecule. Examples of thecompound having the structure represented by the formula (2) include acopolymer of β-pinene and a phenol-based compound and/or a styrene-basedcompound. R² is a divalent residue where two hydrogen atoms are removedfrom benzene ring of a phenol-based compound and/or a styrene-basedcompound.

Examples of the phenol-based compound include phenol, cresol, xylenol,catechol, resorcin, hydroquinone, and bisphenol A. Examples of thestyrene-based compound include styrene, and α-methylstyrene.

In the formula (2), m² represents the degree of polymerization of thestructural unit derived from β-pinene, and is preferably a naturalnumber of 1 to 30. m² is preferably 1 or more, more preferably 2 ormore, and even more preferably 3 or more, and is preferably 30 or less,more preferably 25 or less, and even more preferably 20 or less.

In the formula (2), n² represents the degree of polymerization of therepeating unit consisting of a structural moiety derived from β-pineneand R² bonding to the structural moiety, and is preferably a naturalnumber of 1 to 20. n² is preferably 1 or more, more preferably 2 ormore, and even more preferably 3 or more, and is preferably 20 or less,more preferably 18 or less, and even more preferably 15 or less.

The compound having the structure represented by the formula (3) is apolymer having a structural unit derived from α-pinene, and is morepreferably a polymer consisting of the structural unit derived fromα-pinene.

In the formula (3), m³ represents the degree of polymerization of thestructural unit derived from α-pinene, and is preferably a naturalnumber of 1 to 30. m³ is preferably 1 or more, more preferably 2 ormore, and even more preferably 3 or more, and is preferably 30 or less,more preferably 25 or less, and even more preferably 20 or less.

The compound having the structure represented by the formula (4) is aβ-pinene polymer having a structural unit derived from β-pinene in themolecule, and is more preferably a polymer consisting of the structuralunit derived from β-pinene.

In the formula (4), m⁴ represents the degree of polymerization of thestructural unit derived from β-pinene, and is preferably a naturalnumber of 1 to 30. m⁴ is preferably 1 or more, more preferably 2 ormore, and even more preferably 3 or more, and is preferably 30 or less,more preferably 25 or less, and even more preferably 20 or less.

Particularly preferable examples of (d) the terpene-based resin includeα-pinene-phenol copolymer, α-pinene-α-methylstyrene copolymer,α-pinene-α-methylstyrene-phenol copolymer, β-pinene-phenol copolymer,β-pinene-α-methylstyrene copolymer, and β-pinene-α-methylstyrene-phenolcopolymer. As (d) the terpene-based resin, these copolymers may be usedsolely, or two or more of them may be used in combination.

The softening point of (d) the terpene-based resin is preferably 60° C.or more, more preferably 80° C. or more, and even more preferably 100°C. or more, and is preferably 150° C. or less, more preferably 130° C.or less, and even more preferably 120° C. or less. This is because useof (d) the terpene-based resin having the softening point falling withinthe above range improves the dispersibility of the resin during kneadingthe rubber. It is noted that the softening point of (d) theterpene-based resin is measured with a ring and ball type softeningpoint measuring apparatus according to JIS K 6220-1: 2001, and is atemperature at which the ball drops.

As (d) the terpene-based resin, a commercial product can be used.Examples of the commercial product include Sylvares TP2019 andSylvatraxx 6720 available from Kraton Corporation; and YS RESIN PX 1150Navailable from Yasuhara Chemical Co. Ltd.

The amount of (d) the terpene-based resin is preferably 0.5 part by massor more, more preferably 0.8 part by mass or more, and even morepreferably 1 part by mass or more, and is preferably 10 parts by mass orless, more preferably 8 parts by mass or less, and even more preferably6 parts by mass or less, with respect to 100 parts by mass of (a) thebase rubber. If the amount of the component (d) is less than 0.5 part bymass, the effect of adding the component (d) is small, and thus theimprovement effect on the shot feeling on approach shots may not beobtained. On the other hand, if the amount of the component (d) is morethan 10 parts by mass, the obtained core becomes excessively soft as awhole, and thus the resilience and a travel distance on driver shots maybe lowered. In addition, the shot feeling on approach shots may belowered.

The blending ratio (the component (b)/the component (d)) of thecomponent (b) to the component (d) is preferably 2.0 or more, morepreferably 3.0 or more, and even more preferably 5.0 or more, and ispreferably 35.0 or less, more preferably 30.0 or less, even morepreferably 26.0 or less in a mass ratio. If the blending ratio (thecomponent (b)/the component (d)) of the component (b) to the component(d) falls within the above range, the obtained golf ball has a good shotfeeling on approach shots and travels a great distance on driver shots.

[(e) Organic Sulfur Compound]

The core rubber composition preferably further contains (e) an organicsulfur compound. If (e) the organic sulfur compound is contained, theobtained core has higher resilience.

As (e) the organic sulfur compound, at least one compound selected fromthe group consisting of thiols (thiophenols and thionaphthols),polysulfides, thiurams, thiocarboxylic acids, dithiocarboxylic acids,sulfenamides, dithiocarbamates and thiazoles is preferable.

Examples of the thiols include thiophenols and thionaphthols. Examplesof the thiophenols include thiophenol; thiophenols substituted with afluoro group, such as 4-fluorothiophenol, 2,4-difluorothiophenol,2,5-difluorothiophenol, 2,6-difluorothiophenol,2,4,5-trifluorothiophenol, and 2,4,5,6-tetrafluorothiophenol,pentafluorothiophenol; thiophenols substituted with a chloro group, suchas 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol,2,5-dichlorothiophenol, 2,6-dichlorothiophenol,2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol, andpentachlorothiophenol; thiophenols substituted with a bromo group, suchas 4-bromothiophenol, 2,4-dibromothiophenol, 2,5-dibromothiophenol,2,6-dibromothiophenol, 2,4,5-tribromothiophenol,2,4,5,6-tetrabromothiophenol, and pentabromothiophenol; thiophenolssubstituted with an iodo group, such as 4-iodothiophenol,2,4-diiodothiophenol, 2,5-diiodothiophenol, 2,6-diiodothiophenol,2,4,5-triiodothiophenol, and 2,4,5,6-tetraiodothiophenol,pentaiodothiophenol; and metal salts thereof. As the metal salt, a zincsalt is preferred.

Examples of the thionaphthols (naphthalenethiols) include2-thionaphthol, 1-thionaphthol, 1-chloro-2-thionaphthol,2-chloro-1-thionaphthol, 1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol,1-fluoro-2-thionaphthol, 2-fluoro-1-thionaphthol,1-cyano-2-thionaphthol, 2-cyano-1-thionaphthol, 1-acetyl-2-thionaphthol,2-acetyl-1-thionaphthol, and a metal salt thereof. Preferable examplesinclude 2-thionaphthol, 1-thionaphthol, and a metal salt thereof. Themetal salt is preferably a divalent metal salt, more preferably a zincsalt. Specific examples of the metal salt include the zinc salt of1-thionaphthol and the zinc salt of 2-thionaphthol.

The polysulfides are organic sulfur compounds having a polysulfide bond,

and examples thereof include disulfides, trisulfides, and tetrasulfides.As the polysulfides, diphenylpolysulfides are preferable.

Examples of the diphenylpolysulfides include diphenyldisulfide;diphenyldisulfides substituted with a halogen group, such asbis(4-fluorophenyl)disulfide, bis(2,5-difluorophenyl)disulfide,bis(2,6-difluorophenyl)disulfide, bis(2,4,5-trifluorophenyl)disulfide,bis(2,4,5,6-tetrafluorophenyl)disulfide,bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl)disulfide,bis(2,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide,bis(2,4,5-trichlorophenyl)disulfide,bis(2,4,5,6-tetrachlorophenyl)disulfide,bis(pentachlorophenyl)disulfide, bis(4-bromophenyl)disulfide,bis(2,5-dibromophenyl)disulfide, bis(2,6-dibromophenyl)disulfide,bis(2,4,5-tribromophenyl)disulfide,bis(2,4,5,6-tetrabromophenyl)disulfide, bis(pentabromophenyl)disulfide,bis(4-iodophenyl)disulfide, bis(2,5-diiodophenyl)disulfide,bis(2,6-diiodophenyl)disulfide, bis(2,4,5-triiodophenyl)disulfide,bis(2,4,5,6-tetraiodophenyl)disulfide, andbis(pentaiodophenyl)disulfide; and diphenyldisulfides substituted withan alkyl group, such as bis(4-methylphenyl)disulfide,bis(2,4,5-trimethylphenyl)disulfide, bis(pentamethylphenyl)disulfide,bis(4-t-butylphenyl)disulfide, bis(2,4,5-tri-t-butylphenyl)disulfide,and bis(penta-t-butylphenyl)disulfide.

Examples of the thiurams include thiuram monosulfides such astetramethylthiuram monosulfide; thiuram disulfides such astetramethylthiuram disulfide, tetraethylthiuram disulfide, andtetrabutylthiuram disulfide; and thiuram tetrasulfides such asdipentamethylenethiuram tetrasulfide. Examples of the thiocarboxylicacids include a naphthalene thiocarboxylic acid. Examples of thedithiocarboxylic acids include a naphthalene dithiocarboxylic acid.Examples of the sulfenamides include N-cyclohexyl-2-benzothiazolesulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, andN-t-butyl-2-benzothiazole sulfenamide.

(e) The organic sulfur compound is preferably thiophenols and/or themetal salt thereof, thionaphthols and/or the metal salt thereof,diphenyldisulfides, and thiuramdisulfides, more preferably2,4-dichlorothiophenol, 2,6-difluorothiophenol, 2,6-dichlorothiophenol,2,6-dibromothiophenol, 2,6-diiodothiophenol, 2,4,5-trichlorothiophenol,pentachlorothiophenol, 1-thionaphthol, 2-thionaphthol,diphenyldisulfide, bis(2,6-difluorophenyl)disulfide,bis(2,6-dichlorophenyl)disulfide, bis(2,6-dibromophenyl)disulfide,bis(2,6-diiodophenyl)disulfide, and bis(pentabromophenyl) disulfide.

(e) The organic sulfur compound may be used solely or in combination ofat least two of them.

The amount of (e) the organic sulfur compound is preferably 0.05 part bymass or more, more preferably 0.1 part by mass or more, and even morepreferably 0.2 part by mass or more, and is preferably 5.0 parts by massor less, more preferably 3.0 parts by mass or less, and even morepreferably 2.0 parts by mass or less, with respect to 100 parts by massof (a) the base rubber. If the amount of (e) the organic sulfur compoundis less than 0.05 part by mass, the effect of adding (e) the organicsulfur compound may not be obtained, and thus the resilience of the golfball may not be improved. In addition, if the amount of (e) the organicsulfur compound is more than 5.0 parts by mass, the obtained golf ballmay have an excessively large compression deformation amount, and thusthe resilience thereof may be lowered.

[(f) Metal Compound]

In the case that the core rubber composition contains only theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as theco-crosslinking agent, the core rubber composition preferably furthercontains (f) a metal compound. This is because neutralizing theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with themetal compound in the core rubber composition provides substantially thesame effect as using the metal salt of the α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms as the co-crosslinking agent. It isnoted that in case of using the α,β-unsaturated carboxylic acid having 3to 8 carbon atoms and the metal salt thereof in combination as theco-crosslinking agent, (f) the metal compound may be used as an optionalcomponent.

(f) The metal compound is not particularly limited as long as itneutralizes (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms in the core rubber composition. Examples of (f) the metal compoundinclude a metal hydroxide such as magnesium hydroxide, zinc hydroxide,calcium hydroxide, sodium hydroxide, lithium hydroxide, potassiumhydroxide, and copper hydroxide; a metal oxide such as magnesium oxide,calcium oxide, zinc oxide, and copper oxide; and a metal carbonate suchas magnesium carbonate, zinc carbonate, calcium carbonate, sodiumcarbonate, lithium carbonate, and potassium carbonate. (f) The metalcompound is preferably a divalent metal compound, more preferably a zinccompound. This is because the divalent metal compound reacts with theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, therebyforming a metal crosslinking. Further, use of the zinc compound providesa golf ball with higher resilience.

(f) The metal compound may be used solely or in combination of two ormore of them. In addition, the amount of (f) the metal compound may beappropriately adjusted in accordance with the desired neutralizationdegree of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms.

The core rubber composition used in the present invention may furthercontain additives such as a pigment, a filler for adjusting a weight orthe like, an antioxidant, a peptizing agent, a softening agent or thelike, where necessary.

The filler blended in the core rubber composition is used as a weightadjusting agent for mainly adjusting the weight of the golf ballobtained as a final product. The filler may be blended where necessary.Examples of the filler include an inorganic filler such as zinc oxide,barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, andmolybdenum powder. Zinc oxide is particularly preferably used as thefiller. It is considered that zinc oxide functions as a vulcanizationactivator and increases the hardness of the entire core. The amount ofthe filler is preferably 0.5 part by mass or more, more preferably 1part by mass or more, and is preferably 30 parts by mass or less, morepreferably 25 parts by mass or less, and even more preferably 20 partsby mass or less, with respect to 100 parts by mass of (a) the baserubber. This is because if the amount of the filler is less than 0.5part by mass, it is difficult to adjust the weight, while if the amountof the filler exceeds 30 parts by mass, the weight ratio of the rubbercomponent is reduced and thus the resilience tends to be lowered.

The amount of the antioxidant is preferably 0.1 part by mass or more and1 part by mass or less with respect to 100 parts by mass of (a) the baserubber. In addition, the amount of the peptizing agent is preferably 0.1part by mass or more and 5 parts by mass or less with respect to 100parts by mass of (a) the base rubber.

[Core]

The core of the golf ball according to the present invention can beobtained by mixing and kneading the above-described core rubbercomposition, and molding the kneaded product in a mold. The moldingcondition is not particularly limited, and the molding is generallycarried out at a temperature in a range of from 130° C. to 200° C. undera pressure of 2.9 MPa to 11.8 MPa for 10 minutes to 60 minutes. Forexample, it is preferred that the core rubber composition is heated at130° C. to 200° C. for 10 to 60 minutes, or alternatively the corerubber composition is subjected to a two-step heating, i.e. the corerubber composition is heated at a temperature in a range of from130° C.to 150° C. for 20 to 40 minutes and then heated at a temperature in arange of from 160° C. to 180° C. for 5 to 15 minutes.

The core preferably has a spherical shape. The construction of the coreincludes a single layered structure or a multi-layered structure,preferably includes a single layered structure. Unlike the multi-layeredstructure, the spherical core of the single layered structure does nothave an energy loss at the interface of the multi-layered structure whenhitting, and thus has an enhanced resilience.

The surface hardness (Hs) of the core of the golf ball according to thepresent invention is preferably 60 or more, more preferably 65 or more,even more preferably 70 or more, and is preferably 88 or less, morepreferably 85 or less, and even more preferably 83 or less in Shore Chardness. If the surface hardness (Hs) of the core is 60 or more inShore C hardness, the resilience of the core is better.

In addition, if the surface hardness (Hs) of the core is 85 or less inShore C hardness, the durability is further enhanced.

The center hardness (Ho) of the core is preferably 40 or more, morepreferably 50 or more, and even more preferably 60 or more in Shore Chardness. If the center hardness (Ho) of the core is 40 or more in ShoreC hardness, the core does not become excessively soft, and thus theresilience is better. In addition, the center hardness (Ho) of the coreis preferably 75 or less, more preferably 70 or less, and even morepreferably 68 or less in Shore C hardness. If the center hardness (Ho)is 75 or less in Shore C hardness, the core does not become excessivelyhard, and thus the shot feeling on approach shots is better.

The hardness difference (Hs−Ho) between the surface hardness (Hs) of thecore and the center hardness (Ho) of the core is preferably 5 or more,more preferably 6 or more, and even more preferably 8 or more, and ispreferably 35 or less, more preferably 25 or less, and even morepreferably 20 or less in Shore C hardness. If the hardness difference(Hs−Ho) between the surface hardness (Hs) of the core and the centerhardness (Ho) of the core is 5 or more in Shore C hardness, the obtainedgolf ball has better resilience. In addition, if the hardness difference(Hs−Ho) between the surface hardness (Hs) of the core and the centerhardness (Ho) of the core is 35 or less in Shore C hardness, the golfballs having higher durability are obtained.

The core of the golf ball according to the present invention preferablyhas a diameter of 34.8 mm or more, more preferably 36.8 mm or more, andeven more preferably 38.6 mm or more, and preferably has a diameter of42.2 mm or less, more preferably 41.8 mm or less, even more preferably41.2 mm or less, and most preferably 40.8 mm or less. If the core has adiameter of 34.8 mm or more, the thickness of the intermediate layer andthe cover does not become too thick and thus the resilience becomesbetter. On the other hand, if the core has a diameter of 42.2 mm orless, the thickness of the intermediate layer and the cover does notbecome too thin and thus the intermediate layer and the cover functionbetter.

When the core has a diameter in a range from 34.8 mm to 42.2 mm, thecompression deformation amount of the core (shrinking amount of the corealong the compression direction) when applying a load from an initialload of 98 N to a final load of 1275 N is preferably 2.0 mm or more,more preferably 2.5 mm or more, and even more preferably 3.0 mm or more,and is preferably 5.0 mm or less, more preferably 4.0 mm or less, andeven more preferably 3.5 mm or less. If the compression deformationamount is 2.0 mm or more, the shot feeling becomes better and if thecompression deformation amount is 5.0 mm or less, the resilience becomesbetter.

[Intermediate Layer]

The golf ball according to the present invention comprises a core, andat least one intermediate layer covering the core. The intermediatelayer comprises at least one layer, and thus may be composed of a singlelayer or at least two layers.

In the present invention, the material hardness (Hm) of the intermediatelayer of the golf ball is higher than a material hardness (Hc) of thecover. If the material hardness (Hm) of the intermediate layer is higherthan the material hardness (Hc) of the cover, the golf ball having animproved flight performance on driver shots without lowering thecontrollability on approach shots is obtained. Further, if the materialhardness (Hc) of the cover is lower than the material hardness (Hm) ofthe intermediate layer, the softness of the cover is emphasized, thusthe shot feeling on approach shots is improved. In case of multi-layeredintermediate layers, the material hardness (Hm) of the hardestintermediate layer is higher than the material hardness (Hc) of thecover.

The hardness difference (Hm−Hc) between the material hardness (Hm) ofthe intermediate layer and the material hardness (Hc) of the cover ispreferably 5 or more, more preferably 20 or more, even more preferably30 or more, most preferably 35 or more in Shore D hardness. If thehardness difference (Hm−Hc) is 5 or more in Shore D hardness, the spinrate on approach shots tends to become high while maintaining the flightperformance on driver shots. The upper limit of the hardness difference(Hm−Hc) is, but not particularly limited to, preferably 55, morepreferably 50, even more preferably 45 in Shore D hardness.

The material hardness (Hm) of the intermediate layer is preferably 50 ormore, more preferably 55 or more, even more preferably 65 or more inShore D hardness. If the material hardness (Hm) of the intermediatelayer is less than 50 in Shore D hardness, the sufficient flightdistance on driver shots may not be obtained. The upper limit of thehardness (Hm) of the intermediate layer is, but not particularly limitedto, preferably 85, more preferably 80, even more preferably 75 in ShoreD hardness. The material hardness (Hm) of the intermediate layer is aslab hardness of the intermediate layer composition for forming theintermediate layer molded into a sheet shape. In case of multipleintermediate layers, the material hardness of the intermediate layercomposition constituting each layer may be identical to or differentfrom each other, but it is preferable that the material hardness of allthe intermediate layers falls within the above range.

The thickness of the intermediate layer is preferably 2.5 mm or less,more preferably 2.0 mm or less, even more preferably 1.8 mm or less. Ifthe thickness of the intermediate layer is more than 2.5 mm, the spinrate on approach shots may be reduced, and the shot feeling may becomebad. The thickness of the intermediate layer is preferably 0.5 mm ormore, more preferably 0.8 mm or more, even more preferably 1.0 mm ormore. If the thickness of the intermediate layer is less than 0.5 mm,the durability of the golf ball may be lowered, or the molding of theintermediate layer may become difficult. In case of multipleintermediate layers, the thickness of each intermediate layer may beidentical to or different from the thickness of the other intermediatelayer, as long as a total thickness of all the intermediate layers fallswithin the above range.

[Cover]

The golf ball according to the present invention comprises a covercovering the at least one intermediate layer. The cover constitutes anoutermost layer of the golf ball body except a paint film. In thepresent invention, the material hardness (Hc) of the cover of the golfball is preferably less than 50, more preferably 45 or less, even morepreferably 35 or less, most preferably 33 or less, and is preferably 15or more, more preferably 20 or more, even more preferably 25 or more inShore D hardness. If the material hardness (Hc) of the cover fallswithin the above range, the spin rate on approach shots increases. Thematerial hardness (Hc) of the cover is a slab hardness of the covercomposition for forming the cover molded into a sheet shape.

The thickness of the cove is preferably 1.5 mm or less, more preferably1.0 mm or less, even more preferably 0.8 mm or less. If the thickness ofthe cover is more than 2.0 mm, the spin rate on driver shots mayincrease. The thickness of the cover is preferably 0.3 mm or more, morepreferably 0.4 mm or more, even more preferably 0.5 mm or more. If thethickness of the cover is less than 0.5 mm, the durability of the golfball may be lowered, or the molding of the cover may become difficult.

The total thickness of the intermediate layer and the cover ispreferably 3.6 mm or less, more preferably 3.5 mm or less, even morepreferably 3.4 mm or less. If the total thickness of the intermediatelayer and the cover is more than 3.6 mm, the shot feeling on approachshots may be lowered.

[Materials for the Intermediate Layer and the Cover]

In one embodiment of the present invention, the intermediate layer andthe cover of the golf ball are formed from a composition containing aresin component. The resin component includes a thermoplastic resin orthermosetting resin, preferably includes the thermoplastic resin. In thepresent invention, a resin composition for forming an intermediate layeris sometimes referred to as “intermediate layer composition” and a resincomposition for forming a cover is sometimes referred to as “covercomposition.”

Examples of the thermoplastic resin include an ionomer resin, athermoplastic olefin copolymer, a thermoplastic urethane resin, athermoplastic polyamide resin, a thermoplastic styrene-based resin, athermoplastic polyester resin, and a thermoplastic acrylic resin. Amongthese thermoplastic resins, a thermoplastic elastomer having rubberelasticity is preferable. Examples of the thermoplastic elastomerinclude a thermoplastic polyurethane elastomer, a thermoplasticpolyamide elastomer, a thermoplastic styrene-based elastomer, athermoplastic polyester elastomer, and a thermoplastic acrylic-basedelastomer.

[Ionomer Resin]

Examples of the ionomer resin include an ionomer resin consisting of ametal ion-neutralized product of a binary copolymer composed of anolefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms; an ionomer resin consisting of a metal ion-neutralized product ofa ternary copolymer composed of an olefin, an α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and an α,β-unsaturated carboxylic acidester; and a mixture thereof.

It is noted that, in the present invention, “an ionomer resin consistingof a metal ion-neutralized product of a binary copolymer composed of anolefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms” is sometimes simply referred to as “a binary ionomer resin”, and“an ionomer resin consisting of a metal ion-neutralized product of aternary copolymer composed of an olefin, an α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and an α,β-unsaturated carboxylic acidester” is sometimes simply referred to as “a ternary ionomer resin”.

The olefin is preferably an olefin having 2 to 8 carbon atoms. Examplesof the olefin include ethylene, propylene, butene, pentene, hexene,heptene and octene, and ethylene is particularly preferable. Examples ofthe α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms includeacrylic acid, methacrylic acid, fumaric acid, maleic acid and crotonicacid, and acrylic acid or methacrylic acid is particularly preferable.In addition, as the α,β-unsaturated carboxylic acid ester, methyl ester,ethyl ester, propyl ester, n-butyl ester, isobutyl ester or the like ofacrylic acid, methacrylic acid, fumaric acid, maleic acid or the likecan be used, and acrylic acid ester or methacrylic acid ester isparticularly preferable.

As the binary ionomer resin, a metal ion-neutralized product of anethylene-(meth)acrylic acid binary copolymer is preferable. As theternary ionomer resin, a metal ion-neutralized product of a ternarycopolymer composed of ethylene, (meth)acrylic acid and (meth)acrylicacid ester is preferable. Herein, (meth)acrylic acid means acrylic acidand/or methacrylic acid.

The amount of the α,β-unsaturated carboxylic acid component having 3 to8 carbon atoms in the binary ionomer resin is preferably 15 mass % ormore, more preferably 16 mass % or more, and even more preferably 17mass % or more, and is preferably 30 mass % or less, more preferably 25mass % or less. If the amount of the α,β-unsaturated carboxylic acidcomponent having 3 to 8 carbon atoms is 15 mass % or more, the obtainedconstituent member is easily adjusted to a desired hardness. Inaddition, if the amount of the α,β-unsaturated carboxylic acid componenthaving 3 to 8 carbon atoms is 30 mass % or less, the obtainedconstituent member is not excessively hard and hence has betterdurability and shot feeling.

The neutralization degree of the carboxyl groups of the binary ionomerresin is preferably 15 mole % or more, more preferably 20 mole % ormore, and is preferably 100 mole % or less. If the neutralization degreeis 15 mole % or more, the obtained golf ball has better resilience anddurability. It is noted that the neutralization degree of the carboxylgroups of the binary ionomer resin may be calculated by the followingexpression. In addition, the metal component may be contained in such amanner that the theoretical neutralization degree of the carboxyl groupsof the ionomer resin exceeds 100 mole %.

Neutralization degree of binary ionomer resin (mole %)=100×(mole numberof neutralized carboxyl groups in binary ionomer resin/mole number ofall carboxyl groups in binary ionomer resin)

Examples of the metal ion for neutralizing at least a part of carboxylgroups of the binary ionomer resin include a monovalent metal ion suchas sodium, potassium and lithium; a divalent metal ion such asmagnesium, calcium, zinc, barium and cadmium; a trivalent metal ion suchas aluminum; and other ion such as tin and zirconium.

Specific examples of the binary ionomer resin in terms of trade namesinclude “Himilan (registered trademark) (e.g. Himilan 1555 (Na), Himilan1557 (Zn), Himilan 1605 (Na), Himilan 1706 (Zn), Himilan 1707 (Na),Himilan AM7311 (Mg), Himilan AM7329 (Zn)” available from Mitsui-Du PontPolychemicals Co., Ltd.

Specific examples of the binary ionomer resin in terms of trade namesfurther include “Surlyn (registered trademark) (e.g. Surlyn 8945 (Na),Surlyn 9945 (Zn), Surlyn 8140 (Na), Surlyn 8150 (Na), Surlyn 9120 (Zn),Surlyn 9150 (Zn), Surlyn 6910 (Mg), Surlyn 6120 (Mg), Surlyn 7930 (Li),Surlyn 7940 (Li), Surlyn AD8546 (Li))” available from E.I. du Pont deNemours and Company.

Examples of the ionomer resin available from ExxonMobil ChemicalCorporation include “Iotek (registered trademark) (e.g. Iotek 8000 (Na),Iotek 8030 (Na), Iotek 7010 (Zn), Iotek 7030 (Zn))”.

The above listed binary ionomer resins may be used solely or as amixture of two or more of them. Na, Zn, Li, Mg and the like described inthe parentheses after the trade names indicate metal types ofneutralizing metal ions of the ionomer resins.

The amount of the α,β-unsaturated carboxylic acid component having 3 to8 carbon atoms in the ternary ionomer resin is preferably 2 mass % ormore, more preferably 3 mass % or more, and is preferably 30 mass % orless, more preferably 25 mass % or less.

The neutralization degree of the carboxyl groups of the ternary ionomerresin is preferably 20 mole % or more, more preferably 30 mole % ormore, and is preferably 100 mole % or less. If the neutralization degreeis 20 mole % or more, the golf ball obtained by using the thermoplasticresin composition has better resilience and durability. It is noted thatthe neutralization degree of the carboxyl groups of the ionomer resinmay be calculated by the following expression. In addition, the metalcomponent may be contained in such a manner that the theoreticalneutralization degree of the carboxyl groups of the ionomer resinexceeds 100 mole %.

Neutralization degree of ionomer resin (mole %)32 100×(mole number ofneutralized carboxyl groups in ionomer resin/mole number of all carboxylgroups in ionomer resin)

Examples of the metal ion for neutralizing at least a part of carboxylgroups of the ternary ionomer resin include a monovalent metal ion suchas sodium, potassium and lithium; a divalent metal ion such asmagnesium, calcium, zinc, barium and cadmium; a trivalent metal ion suchas aluminum; and other ion such as tin and zirconium.

Specific examples of the ternary ionomer resin in terms of trade namesinclude “Himilan (e.g. Himilan AM7327 (Zn), Himilan 1855 (Zn), Himilan1856 (Na), Himilan AM7331 (Na))” available from Mitsui-Du PontPolychemicals Co., Ltd. Further, examples of the ternary ionomer resinavailable from E.I. du Pont de Nemours and Company include “Surlyn 6320(Mg), Surlyn 8120 (Na), Surlyn 8320 (Na), Surlyn 9320 (Zn), Surlyn 9320W(Zn), HPF1000 (Mg), HPF2000 (Mg), etc.)”. In addition, Examples of theternary ionomer resin available from ExxonMobil Chemical Corporationinclude “Iotek 7510 (Zn), Iotek 7520 (Zn), etc.)”. It is noted that Na,Zn, Mg and the like described in the parentheses after the trade namesindicate types of neutralizing metal ions. The ternary ionomer resin maybe used solely, or two or more of them may be used in combination.

[Thermoplastic Olefin Copolymer]

Examples of the thermoplastic olefin copolymer include a binarycopolymer composed of an olefin and an α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms; a ternary copolymer composed of an olefin,an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and anα,β-unsaturated carboxylic acid ester; and a mixture thereof. Thethermoplastic olefin copolymer is a non-ionic copolymer in which thecarboxylic groups thereof are not neutralized.

It is noted that, in the present invention, “a binary copolymer composedof an olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms” is sometimes simply referred to as “a binary copolymer”, and “aternary copolymer composed of an olefin, an α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and an α,β-unsaturated carboxylic acidester” is sometimes simply referred to as “a ternary copolymer”.

Examples of the olefin include those listed as the olefin constitutingthe ionomer resin. In particular, the olefin is preferably ethylene.Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms and the ester thereof include those listed as the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and the ester thereofconstituting the ionomer resin.

As the binary copolymer, a binary copolymer composed of ethylene and(meth)acrylic acid is preferable. As the ternary copolymer, a ternarycopolymer composed of ethylene, (meth)acrylic acid and (meth)acrylicacid ester is preferable. Herein, (meth)acrylic acid means acrylic acidand/or methacrylic acid.

The amount of the α,β-unsaturated carboxylic acid component having 3 to8 carbon atoms in the binary copolymer or ternary copolymer ispreferably 4 mass % or more, more preferably 5 mass % or more, and ispreferably 30 mass or less, more preferably 25 mass % or less.

Specific examples of the binary copolymer in terms of trade namesinclude an ethylene-methacrylic acid copolymer in a trade name of“NUCREL (registered trademark) (e.g. “NUCREL N1050H”, “NUCREL N2050H”,“NUCREL N1110H”, “NUCREL NO200H”)” available from Mitsui-Du PontPolychemicals Co., Ltd.; and an ethylene-acrylic acid copolymer in atrade name of “PRIMACOR (registered trademark) 5980I” available from DowChemical Corporation.

Specific examples of the ternary copolymer in terms of trade namesinclude trade name “NUCREL (e.g. “NUCREL AN4318” “NUCREL AN4319”)”available from Mitsui-Du Pont Polychemicals Co., Ltd.; trade name“NUCREL (e.g. “NUCREL AE”)” available from E.I. du Pont de Nemours andCompany; and trade name “PRIMACOR (e.g. “PRIMACOR AT310”, “PRIMACORAT320”)” available from Dow Chemical Corporation. The binary copolymeror ternary copolymer may be used solely, or two or more of them may beused in combination.

[Thermoplastic Polyurethane Resin and Thermoplastic PolyurethaneElastomer]

Examples of the thermoplastic polyurethane resin and the thermoplasticpolyurethane elastomer include a thermoplastic resin and a thermoplasticelastomer which have a plurality of urethane bonds in the main chain ofthe molecule. As the polyurethane, a product obtained by a reactionbetween a polyisocyanate component and a polyol component is preferable.Examples of the thermoplastic polyurethane elastomer include trade names“Elastollan (registered trademark) XNY85A”, “Elastollan XNY90A”,“Elastollan XNY97A”, “Elastollan ET885” and “Elastollan ET890” availablefrom BASF Japan Ltd.

[Thermoplastic Styrene-Based Elastomer]

As the thermoplastic styrene-based elastomer, a thermoplastic elastomercontaining a styrene block is suitably used. The styreneblock-containing thermoplastic elastomer has a polystyrene block whichis a hard segment, and a soft segment. The typical soft segment is adiene block. Examples of the constituent component of the diene blockinclude butadiene, isoprene, 1,3-pentadiene and2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferable. Twoor more of the constituent components may be used in combination.

Examples of the thermoplastic elastomer containing the styrene blockinclude a styrene-butadiene-styrene block copolymer (SBS), astyrene-isoprene-styrene block copolymer (SIS), astyrene-isoprene-butadiene-styrene block copolymer (SIBS), ahydrogenated product of SBS, a hydrogenated product of SIS, and ahydrogenated product of SIBS. Examples of the hydrogenated product ofSBS include a styrene-ethylene-butylene-styrene block copolymer (SEBS).Examples of the hydrogenated product of SIS include astyrene-ethylene-propylene-styrene block copolymer (SEPS). Examples ofthe hydrogenated product of SIBS include astyrene-ethylene-ethylene-propylene-styrene block copolymer (SEEPS).

The amount of the styrene component in the thermoplastic elastomercontaining the styrene block is preferably 10 mass % or more, morepreferably 12 mass % or more, and most preferably 15 mass % or more.From the viewpoint of the shot feeling of the obtained golf ball, theabove amount is preferably 50 mass % or less, more preferably 47 mass %or less, and most preferably 45 mass % or less.

Examples of the thermoplastic elastomer containing the styrene blockinclude an alloy of one member or at least two members selected from thegroup consisting of SBS, SIS, SIBS, SEBS, SEPS, SEEPS and hydrogenatedproducts thereof with a polyolefin. It is estimated that the olefincomponent in the alloy contributes to the improvement in compatibilitywith the ionomer resin. If the alloy is used, the resilience performanceof the golf ball is enhanced. An olefin having 2 to 10 carbon atoms ispreferably used. Suitable examples of the olefin include ethylene,propylene, butene and pentene. Ethylene and propylene are particularlypreferable.

Specific examples of the polymer alloy include “TEFABLOC (registeredtrademark) T3221C”, “TEFABLOC T3339C”, “TEFABLOC SJ4400N”, “TEFABLOCSJ5400N”, “TEFABLOC SJ6400N”, “TEFABLOC SJ7400N”, “TEFABLOC SJ8400N”,“TEFABLOC SJ9400N” and “TEFABLOC SR04” available from MitsubishiChemical Corporation. Other specific examples of the thermoplasticelastomer containing the styrene block include “Epofriend A1010”available from Daicel Chemical Industry Co., Ltd., and “SEPTON HG-252”available from Kuraray Co., Ltd.

[Thermoplastic Polyamide Resin and Thermoplastic Polyamide Elastomer]

The thermoplastic polyamide is not particularly limited as long as it isa thermoplastic resin having a plurality of amide bonds (—NH—CO—) in themain chain of the molecule, and examples thereof include a producthaving amide bonds formed in the molecule through ring-openingpolymerization of a lactam, or a reaction between a diamine componentand a dicarboxylic acid component.

Examples of the polyamide resin include an aliphatic polyamide such aspolyamide 6, polyamide 11, polyamide 12, polyamide 66, polyamide 610,polyamide 6T, polyamide 61, polyamide 9T, polyamide M5T and polyamide612; an aromatic polyamide such as poly-p-phenylene terephthalamide andpoly-m-phenylene isophthalamide. These polyamides may be used solely, ortwo or more of them may be used in combination. Among them, thealiphatic polyamide such as polyamide 6, polyamide 66, polyamide 11 andpolyamide 12 is preferable.

Specific examples of the polyamide resin in terms of trade names include“Rilsan (registered trademark) B (e.g. Rilsan BESN TL, Rilsan BESN P20TL, Rilsan BESN P40 TL, Rilsan MB3610, Rilsan BMF O, Rilsan BMN O,Rilsan BMN O TLD, Rilsan BMN BK TLD, Rilsan BMN P20 D, Rilsan BMN P40D)” available from Arkema K.K.

The polyamide elastomer has a hard segment portion composed of apolyamide component, and a soft segment portion. Examples of the softsegment portion of the polyamide elastomer include a polyether estercomponent and a polyether component. Examples of the polyamide elastomerinclude a polyether ester amide obtained by a reaction between apolyamide component (hard segment component) and a polyether estercomponent (soft segment component) which is formed from apolyoxyalkylene glycol and a dicarboxylic acid; and a polyether amideobtained by a reaction between a polyamide component (hard segmentcomponent) and a polyether component (soft segment component) which isformed from a dicarboxylic acid or diamine and a compound obtained byaminating or carboxylating both terminals of a polyoxyalkylene glycol.

Examples of the polyamide elastomer include “PEBAX (registeredtrademark) 2533”, “PEBAX 3533”, “PEBAX 4033” and “PEBAX 5533” availablefrom Arkema K.K.

[Thermoplastic Polyester Resin and Thermoplastic Polyester Elastomer]

The thermoplastic polyester resin is not particularly limited as long asit has a plurality of ester bonds in the main chain of the molecule, andpreferable examples thereof include a product obtained by a reactionbetween a dicarboxylic acid and a diol. Examples of the thermoplasticpolyester elastomer include a block copolymer having a hard segmentcomposed of a polyester component, and a soft segment. Examples of thepolyester component constituting the hard segment include an aromaticpolyester. Examples of the soft segment component include an aliphaticpolyether and an aliphatic polyester.

Specific examples of the polyester elastomer include “Hytrel (registeredtrademark) 3548” and “Hytrel 4047” available from Du Pont-Toray Co.,Ltd., and “Primalloy (registered trademark) A1606”, “Primalloy B1600”and “Primalloy B1700” available from Mitsubishi Chemical Corporation.

[Thermoplastic (meth)acrylic-Based Elastomer]

Examples of the thermoplastic (meth)acrylic-based elastomer include athermoplastic elastomer obtained by copolymerizing ethylene and(meth)acrylic acid ester. Specific examples of the thermoplastic(meth)acrylic-based elastomer include “KURARITY (a block copolymer ofmethyl methacrylate and butyl acrylate)” available from Kuraray Co.,Ltd.

The intermediate layer composition and the cover composition preferablycontain an ionomer resin as a resin component. The intermediate layercomposition and the cover composition may consist of an ionomer resin,or may further contain another resin in addition to the ionomer resin.

The intermediate layer composition and the cover composition may furthercontain additives. Examples of the additives include a pigment componentsuch as a white pigment (e.g. titanium oxide) and a blue pigment, aweight adjusting agent, a dispersant, an antioxidant, an ultravioletabsorber, a light stabilizer, a fluorescent material or fluorescentbrightener. Examples of the weight adjusting agent include inorganicfillers such as zinc oxide, barium sulfate, calcium carbonate, magnesiumoxide, tungsten powder, and molybdenum powder.

The amount of the white pigment (e.g. titanium oxide) in the covercomposition is preferably 0.5 part by mass or more, more preferably 1part by mass or more, and is preferably 10 parts by mass or less, morepreferably 8 parts by mass or less, with respect to 100 parts by mass ofthe resin component constituting the cover. This is because if theamount of the white pigment is 0.5 part by mass or more, it is possibleto impart the opacity to the cover, and if the amount of the whitepigment is more than 10 parts by mass, the durability of the obtainedcover may deteriorate.

Examples of the method for molding the intermediate layer of the golfball of the present invention include a method which comprises moldingthe intermediate layer composition into a half hollow-shell, coveringthe spherical body with two of the half shells, and subjecting thespherical body with two of the half hollow-shells to the compressionmolding; and a method which comprises injection molding the intermediatelayer composition directly onto the spherical body.

In case of injection molding the intermediate layer composition onto thespherical body to form the intermediate layer, it is preferred to useupper and lower molds, each having a hemispherical cavity. When moldingthe intermediate layer by the injection molding method, the hold pin isprotruded to hold the spherical body, and the intermediate layercomposition which has been heated and melted is charged and then cooledto form the intermediate layer.

When molding the intermediate layer by the compression molding method,the molding of the half shell may be performed by either a compressionmolding method or an injection molding method, and the compressionmolding method is preferred. Compression molding the intermediate layercomposition into the half shell may be carried out, for example, under apressure of 1 MPa or more and 20 MPa or less at a temperature of −20° C.or more and +70° C. or less relative to the flow beginning temperatureof the intermediate layer composition. If the molding is carried outunder the above conditions, the half shell having a uniform thicknesscan be formed. Examples of the method for molding the intermediate layerby using the half shell include a method of covering the spherical bodywith two of the half shells, and compression molding the spherical bodywith two of the half shells. Compression molding the half shells intothe intermediate layer may be carried out, for example, under a pressureof 0.5 MPa or more and 25 MPa or less at a temperature of −20° C. ormore and +70° C. or less relative to the flow beginning temperature ofthe intermediate layer composition. If the molding is carried out underthe above conditions, the intermediate layer having a uniform thicknesscan be formed.

It is noted that the molding temperature means the highest temperaturewhere the temperature at the surface of the concave portion of the lowermold reaches from closing the mold to opening the mold. In addition, theflow beginning temperature of the composition may be measured using thethermoplastic resin composition in a pellet form under the followingconditions with “Flow Tester CFT-500” available from ShimadzuCorporation.

Measuring conditions: Plunger area: 1 cm², Die length: 1 mm, Diediameter: 1 mm, Load: 588.399 N, Starting temperature: 30° C., andTemperature rising rate: 3° C./min.

Examples of the method for molding the cover of the golf ball of thepresent invention include a method which comprises molding the covercomposition into a hollow-shell, covering the core with a plurality ofthe hollow-shells and subjecting the core with a plurality of the hollowshells to the compression molding (preferably a method which comprisesmolding the cover composition into a half hollow-shell, covering thecore with two of the half hollow-shells, and subjecting the core withtwo of the half hollow-shells to the compression molding); and a methodwhich comprises injection molding the cover composition directly ontothe core. The golf ball body having the cover formed thereon is ejectedfrom the mold, and as necessary, the golf ball body is preferablysubjected to surface treatments such as deburring, cleaning, andsandblast. In addition, if desired, a mark may be formed.

When molding the cover, concave portions called “dimple” are usuallyformed on the surface of the cover. The total number of dimples formedon the cover is preferably 200 or more and 500 or less. If the totalnumber is less than 200, the dimple effect is hardly obtained. On theother hand, if the total number exceeds 500, the dimple effect is hardlyobtained because the size of the respective dimples is small. The shape(shape in a plan view) of dimples includes, for example, withoutlimitation, a circle, a polygonal shape such as a roughly triangularshape, a roughly quadrangular shape, a roughly pentagonal shape, aroughly hexagonal shape, and other irregular shape. The shape of dimplesis employed solely or at least two of them may be used in combination.

The golf ball body having the cover formed thereon is ejected from themold, and is preferably subjected to surface treatments such asdeburring, cleaning and sandblast where necessary. In addition, ifdesired, a paint film or a mark may be formed.

[Golf Ball]

The golf ball construction of the present invention is not particularlylimited, as long as the golf ball comprises a core and at least onelayer of an intermediate layer and at least one cover covering theintermediate layer. The FIGURE is a partially cutaway cross-sectionalview of a golf ball according to one embodiment of the presentinvention. The golf ball 2 comprises a core 104, and an intermediatelayer 106 covering the core 104, and a cover 112 covering theintermediate layer 106. A plurality of dimples 114 are formed on thesurface of the cover. Other portion than the dimples 114 on the surfaceof the golf ball is a land 116. The golf ball is provided with a paintlayer and a mark layer outside the cover, but these layers are notdepicted.

The construction of the golf ball includes, but is not particularlylimited to, for example, a three-piece golf ball composed of a singlelayered core and a single layered intermediate layer disposed around thecore, and a single layered cover around the intermediate layer; afour-piece golf ball composed of a single layered core, two intermediatelayers disposed around the core, and a single layered cover disposedaround the intermediate layer; a multi-piece golf ball such as five ormore piece composed of a single layered core, three or more intermediatelayers disposed around the core and a single layered cover around theintermediate layer. The present invention is suitably applied to any oneof the golf balls having the above construction.

The golf ball of the present invention preferably has a diameter in arange from 40 mm to 45 mm. In light of satisfying a regulation of USGolf Association (USGA), the diameter is particularly preferably 42.67mm or more. In light of prevention of air resistance, the diameter ismore preferably 44 mm or less, particularly preferably 42.80 mm or less.In addition, the golf ball of the present invention preferably has amass of 40 g or more and 50 g or less. In light of obtaining greaterinertia, the mass is more preferably 44 g or more, particularlypreferably 45.00 g or more. In light of satisfying a regulation of USGA,the mass is particularly preferably 45.93 g or less.

When the golf ball of the present invention has a diameter in a rangefrom 40 mm to 45 mm, the compression deformation amount of the golf ball(shrinking amount of the golf ball along the compression direction) whenapplying a load from 98 N as an initial load to 1275 N as a final loadto the golf ball is preferably 2.0 mm or more, more preferably 2.3 mm ormore, and even more preferably 2.5 mm or more, and is preferably 4.0 mmor less, more preferably 3.8 mm or less, and even more preferably 3.5 mmor less. If the compression deformation amount is 2.0 mm or more, thegolf ball does not become excessively hard, and thus the shot feelingthereof becomes better. On the other hand, if the compressiondeformation amount is 4.0 mm or less, the resilience of the golf ballbecomes higher.

EXAMPLES

Next, the present invention will be described in detail by way ofexamples. However, the present invention is not limited to the examplesdescribed below. Various changes and modifications without departingfrom the spirit of the present invention are included in the scope ofthe present invention.

[Evaluation Methods] (1) Compression Deformation of Amount

A compression deformation amount of the core or golf ball (a shrinkingamount of the core or golf ball along the compression direction), whenapplying a load from an initial load of 98 N to a final load of 1275 Nto the core or golf ball, was measured.

(2) Core Hardness (Shore C Hardness)

The core hardness was measured with an automatic hardness tester(Digitest II available from Bareiss company). The testing device was“Shore C”. The hardness at the surface of the core was adopted as thesurface hardness of the core. In addition, the core was cut into twohemispheres to obtain a cut plane, and the hardness at the central pointof the cut plane was measured.

(3) Slab Hardness (Shore D Hardness)

Sheets with a thickness of about 2 mm were produced by heat pressing theintermediate layer composition or the cover composition. The sheets werestored at 23° C. for two weeks. At least three of these sheets werestacked on one another so as not to be affected by the measuringsubstrate on which the sheets were placed, and the hardness of the stackwas measured with an automatic hardness tester (Digitest II, availablefrom Bareiss company) using a testing device of “Shore D”.

(4) Spin Rate on about 20-Yard Approach Shots

A sand wedge (trade name: “CG 15 forged wedge”, loft angel: 52°,available from Cleveland Golf) was installed on a swing robot M/Cavailable from Golf Laboratories, Inc. The golf ball was hit at a headspeed of 16 m/sec and the spin rate (rpm) thereof was measured. Asequence of photographs of the hit golf ball were taken for measuringthe spin rate. The measurement was conducted ten times for each golfball, and the average value thereof was calculated respectively. Thespin rate was shown as a difference from the spin rate of the golf ballNo. 10.

(5) Shot Feeling

An actual hitting test was carried out by twenty golfers. Twenty golfersaim the place at a distance of 20 yards and hit the golf ball. Based ona number of golfers who felt the shot feeling not too soft and not toohard, the shot feeling was evaluated in accordance with the followinggrading standard.

Grading Standard

-   -   E (excellent): 16 or more golfers    -   G (good): 10 or more and 15 or less golfers    -   F (Fair): 5 or more and 9 or less golfers    -   P (Poor): 4 or less golfers

[Production of Golf Ball] (1) Production of Core

According to the formulations shown in Table 1, the rubber compositionswere kneaded with a kneading roll, and heat pressed in upper and lowermolds, each having a hemispherical cavity, at a temperature of 170° C.for 20 minutes to obtain spherical cores. It is noted that bariumsulfate was added in an appropriate amount such that the obtained golfballs had a mass of 45.6 g.

TABLE 1 Rubber composition No. A B C D E F Polybutadiene 100 100 100 100100 100 Zinc oxide 5 5 5 5 5 5 Zinc acrylate 34.1 36.0 37.9 41.3 28.544.9 Barium sulfate * * * * * * PBDS 0.6 0.6 0.6 0.6 0.6 0.6 Dicumylperoxide 0.8 0.8 0.8 0.8 0.8 0.8 TP2019 1 3 5 10 0 15 VulcanizationTemperature 170 170 170 170 170 170 Condition (° C.) Time (min) 20 20 2020 20 20 * Appropriate amount Formulation: parts by mass

The materials used in Table 1 are shown below.

Polybutadiene: high-cis polybutadiene rubber BR730 (amount ofcis-1,4-bond=95 mass %, amount of 1,2-vinyl bond=1.3 mass %, Moonyviscosity (ML₁₊₄ (100° C.)=55, molecular weight distribution (Mw/Mn)=3)available from JSR Corporation

Zinc oxide: “Ginrei R” available from Toho Zinc Co., Ltd.

Zinc acrylate: ZN-DA90S: zinc acrylate available from Nisshoku TechnoFine Chemical Co., Ltd.

Barium sulfate: “Barium Sulfate BD” available from Sakai ChemicalIndustry Co., Ltd.

PBDS: bis(pentabromophenyl) disulfide available from Kawaguchi ChemicalIndustry Co., Ltd.

Dicumyl peroxide: available from Tokyo Chemical Industry Co., Ltd.

TP2019 (Sylvares TP2019): pinene-phenol copolymer (softening point: 125°C.) available from KRATON CORPORATION

(2) Preparation of Intermediate Layer Composition and Cover Composition

According to the formulations shown in Table 2 and Table 3, thematerials were mixed with a twin-screw kneading extruder to prepare aresin composition (an intermediate layer composition or a covercomposition) in a pellet form. The extruding conditions were a screwdiameter of 45 mm, a screw rotational speed of 200 rpm, and screwL/D=35, and the mixture was heated to 160 to 240° C. at the die positionof the extruder.

TABLE 2 Intermediate layer Resin composition No. a b c d Himilan AM7337— — 26 — Himilan AM 7329 25 50 26 — Himilan 1605 — 47 — — Himilan 1555 —— — 47 Himilan 1557 — — — 46 Surlyn 8150 50 — — — Surlyn 9150 25 — — —TEFABLOC T3221C — 3 48 7 Titanium dioxide  4 4 4 4 JF-90   0.2 0.2 0.20.2 Slab hardness (Shore D) 68 63 35 57 Formulation: parts by mass

TABLE 3 Cover resin composition No. d e f g Elastollan NY80A — 100 — —Elastollan NY82A — — 100 — Elastollan NY90A — — — 90 Elastollan NY97A —— — 10 Himilan 1555 47 — — — Himilan 1557 46 — — — TEFABLOC T3221C 7 — —— Titanium dioxide 4 4 4 4 JF-90 0.2 0.2 0.2 0.2 Slab hardness (Shore D)57 27 29 40 Formulation: parts by mass

The materials used in Table 2 and Tabl3 3 are shown below.

Himilan AM7337: Sodium ion-neutralized ethylene-methacrylic acidcopolymer ionomer resin available from Du Pont-Mitsui Polychemicals Co.,Ltd.

Himilan AM7329: Zinc ion-neutralized ethylene-methacrylic acid copolymerionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd.

Himilan 1605: Sodium ion-neutralized ethylene-methacrylic acid copolymerionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd.

Himilan 1555: Sodium ion-neutralized ethylene-methacrylic acid copolymerionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd.

Himilan 1557: Zinc ion-neutralized ethylene-methacrylic acid copolymerionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd.

Surlyn 8150: Sodium ion-neutralized ethylene-methacrylic acid copolymerionomer resin available from Du Pont de Nemours, Inc.

Surlyn 9150: Zinc ion-neutralized ethylene-methacrylic acid copolymerionomer resin available from Du Pont de Nemours, Inc.

TEFABLOC T3221C: thermoplastic styrene elastomer available fromMitsubishi Chemical Corporation

Elastollan NY80A: thermoplastic polyurethane elastomer with Shore Ahardness of 80, available from BASF Japan Ltd.

Elastollan NY82A: thermoplastic polyurethane elastomer with Shore Ahardness of 82, available from BASF Japan Ltd.

Elastollan NY90A: thermoplastic polyurethane elastomer with Shore Ahardness of 90, available from BASF Japan Ltd.

Elastollan NY97A: thermoplastic polyurethane elastomer with Shore Ahardness of 97, available from BASF Japan Ltd.

A-220: titanium dioxide available from Ishihara Sangyo Kaisha, Ltd.

JF-90: Light stabilizer available from Johoku chemical Co., Ltd.

(3) Preparation of Golf Ball

The intermediate layer composition was injected around the sphericalcore to obtain a spherical body having an intermediate layer. Theobtained spherical body was charged in a final mold composed of upperand lower molds, each having a hemispherical cavity. This final mold hada plurality of pimples on the cavity surface. The cover composition wasfilled around the spherical body by an injection molding method to moldthe cover. Dimples having an inverted shape of the pimples were formedon the cover. The evaluation results of the obtained golf balls areshown in Table 4 and Table 5.

TABLE 4 Golf ball No. 1 2 3 4 5 6 7 8 9 Core Formulation No. A B C D B BB B B Compression deformation amount (mm) 3.26 3.22 3.19 3.13 3.22 3.213.22 3.18 3.22 Diameter (mm) 39.7 39.7 39.7 39.7 39.7 39.3 39.7 38.139.7 Center hardness Ho (Shore C) 66.8 67.3 67.6 68.5 67.3 67.3 67.367.3 67.3 Surface hardness Hs (Shore C) 82.1 80.6 77.2 74.5 80.6 80.480.6 80.0 80.6 Hardness difference Hs − Ho (Shore C) 15.3 13.3 9.6 6.013.3 13.1 13.3 12.7 13.3 Intermediate Formulation No. a a a a a a b a dlayer Thickness (mm) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.6 1.0 Hardness Hm(Shore D) 68 68 68 68 68 68 63 68 57 Cover Formulation No. e e e e g f de g Thickness (mm) 0.5 0.5 0.5 0.5 0.5 0.7 0.5 0.7 0.5 Hardness Hc(Shore D) 27 27 27 27 40 29 57 27 40 Ball Compression deformation amount(mm) 2.77 2.77 2.77 2.77 2.75 2.77 2.72 2.68 2.75 Hardness difference Hm− Hc (Shore D) 41 41 41 41 28 39 6 41 17 Approach spin rate (rpm) 130160 180 230 80 145 30 240 80 Shot feeling G G E G G E G G G

TABLE 5 Golf ball No. 10 11 12 13 14 15 Core Formulation No. E F B B C DCompression deformation amount (mm) 3.28 3.08 3.22 3.22 3.19 3.13Diameter (mm) 39.7 39.7 39.7 39.7 39.7 39.7 Center hardness Ho (Shore C)64.2 69.2 67.3 67.3 67.6 68.5 Surface hardness Hs (Shore C) 83.6 71.680.6 80.6 77.2 74.5 Hardness difference Hs − Ho (Shore C) 19.4 2.4 13.313.3 9.6 6.0 Intermediate Formulation No. a a c c c c layer Thickness(mm) 1 1 1 1 1 1 Hardness Hm (Shore D) 68 68 35 35 35 35 CoverFormulation No. e e g d d d Thickness (mm) 0.5 0.5 0.5 0.5 0.5 0.5Hardness Hc (Shore D) 27 27 40 57 57 57 Ball Compression deformationamount (mm) 2.77 2.77 2.83 2.77 2.77 2.77 Hardness difference Hm − Hc(Shore D) 41 41 −5 −22 −22 −22 Approach spin rate (rpm) 0 250 −40 −250−210 −160 Shot feeling F P F P P F

As apparent from the results of Table 4, the golf balls comprising acore, at least one intermediate layer covering the core and a covercovering the intermediate layer, wherein the core is formed from a corerubber composition containing (a) a base rubber containing apolybutadiene, (b) an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c)a crosslinking initiator, and (d) a terpene-based resin, and a materialhardness (Hm) of the intermediate layer is higher than a materialhardness (Hc) of the cover, and the material hardness (Hc) of the coveris less than 50 in Shore D hardness has high spin rate and a good shotfeeling on approach shots.

The golf ball according to the present invention has high spin rate anda good shot feeling on approach shots. This application is based onJapanese patent application No. 2018-242104 filed on Dec. 26, 2018, thecontent of which is hereby incorporated by reference.

1. A golf ball comprising a core, at least one intermediate layercovering the core and a cover covering the intermediate layer, whereinthe core is formed from a core rubber composition containing (a) a baserubber containing a polybutadiene, (b) an α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and/or a metal salt thereof as aco-crosslinking agent, (c) a crosslinking initiator, and (d) aterpene-based resin, and a material hardness (Hm) of the intermediatelayer is higher than a material hardness (Hc) of the cover, and thematerial hardness (Hc) of the cover is less than 50 in Shore D hardness.2. The golf ball according to claim 1, wherein (d) the terpene-basedresin is at least one member selected from the group consisting of aterpene polymer, a terpene-phenol copolymer, a terpene-styrenecopolymer, a terpene-phenol-styrene copolymer, a hydrogenatedterpene-phenol copolymer, a hydrogenated terpene-styrene copolymer, anda hydrogenated terpene-phenol-styrene copolymer.
 3. The golf ballaccording to claim 1, wherein the terpene-based resin is at least onemember selected from the group consisting of compounds having astructure represented by the following formulae (1) to (4):

wherein in the formulae (1) to (4), R¹ and R² each independentlyrepresent a divalent residue of a phenol-based compound and/or astyrene-based compound, m¹ to m⁴ each independently represent a naturalnumber of 1 to 30, and n¹ to n² each independently represent a naturalnumber of 1 to
 20. 4. The golf ball according to claim 1, wherein (d)the terpene-based resin is at least one member selected from the groupconsisting of an α-pinene-phenol copolymer, an α-pinene-α-methylstyrenecopolymer, an α-pinene-α-methylstyrene-phenol copolymer, aβ-pinene-phenol copolymer, a β-pinene-α-methylstyrene copolymer, and aβ-pinene-α-methylstyrene-phenol copolymer.
 5. The golf ball according toclaim 1, wherein (d) the terpene-based resin has a softening point offrom 60° C. to 150° C.
 6. The golf ball according to claim 1, whereinthe core rubber composition contains (d) the terpene-based resin in anamount of from 0.5 part by mass to 10 parts by mass with respect to 100parts by mass of (a) the base rubber.
 7. The golf ball according toclaim 1, wherein a blending ratio (component (b)/component (d)) of thecomponent (b) to the component (d) ranges from 2.0 to 35.0 in a massratio.
 8. The golf ball according to claim 1, wherein the polybutadieneincludes a high-cis polybutadiene having a cis-1,4 bond in an amount of90 mass % or more.
 9. The golf ball according to claim 1, wherein asurface hardness (Hs) of the core ranges from 60 to 88 in Shore Chardness.
 10. The golf ball according to claim 1, wherein a centralhardness (Ho) of the core ranges from 40 to 75 in Shore C hardness. 11.The golf ball according to claim 1, wherein a hardness difference(Hs−Ho) between a surface hardness (Hs) of the core and a centralhardness (Ho) of the core ranges from 5 to 35 in Shore C hardness. 12.The golf ball according to claim 2, wherein the terpene-phenol copolymerhas an acid value in a range of from 10 mgKOH/g 300 mgKOH/g.
 13. Thegolf ball according to claim 2, wherein the terpene-phenol copolymer hasa hydroxy value in a range of from 30 mgKOH/g to 150 mgKOH/g.
 14. Thegolf ball according to claim 1, wherein a hardness difference (Hm−Hc)between the material hardness (Hm) of the intermediate layer and thematerial hardness (Hc) of the cover is 5 or more in Shore D hardness.15. A golf ball comprising a core, at least one intermediate layercovering the core and a cover covering the intermediate layer, whereinthe core is formed from a core rubber composition containing (a) a baserubber containing a polybutadiene, (b) an α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and/or a metal salt thereof as aco-crosslinking agent, (c) a crosslinking initiator, and (d) aterpene-based resin, and a material hardness (Hm) of the intermediatelayer is higher than a material hardness (Hc) of the cover, and thematerial hardness (Hc) of the cover is less than 50 in Shore D hardness,the core rubber composition contains (d) the terpene-based resin in anamount of from 0.5 part by mass to 10 parts by mass with respect to 100parts by mass of (a) the base rubber, and (d) the terpene-based resin isat least one member selected from the group consisting of anα-pinene-phenol copolymer, an α-pinene-α-methylstyrene copolymer, anα-pinene-α-methylstyrene-phenol copolymer, a β-pinene-phenol copolymer,a β-pinene-α-methylstyrene copolymer, and aβ-pinene-α-methylstyrene-phenol copolymer.